The present invention relates to a video disc player. More particularly, the invention relates to a device for eliminating time base variations in video disc players.
For reproduction of stable and normal color pictures in a video disc player, time base variations in color playedback video signals should be reduced to 10 ns or less. However, a video disc cannot be set on a turntable without some eccentricity. This inherently results in time base variations, for example, as much as 20 .mu.s for color video signals reproduced during one revolution of the video disc.
Thus, a device for eliminating time base variations must be provided in video disc player. For this purpose, a tangential servo mechanism including a tangential mirror for deflecting a reading beam in the direction of a line tangential to the direction of disc rotation has been used in optical-type video disc players. The tangential servo mechanism uses a color burst signal which is present in color video signals as phase information in playedback signals. More specifically, a position on a time base of either the leading or trailing edge of a reproduced color burst which appears about 7 .mu.s following the leading edge of a horizontal synchronizing signal is utilized as phase information for the color video signals.
Such a prior-art device for eliminating time base variations is shown, as an example, in FIG. 1, with waveforms of operating signals therein indicated in FIG. 2. In this conventional device, a color video signal A (FIG. 2A) is fed to a terminal 1. This signal is produced by a circuit (not shown) for detecting color video signals recorded on a video disc set on a video disc player. A synchronizing pulse separator 2 separates the horizontal synchronizing pulse signal from the color video signal A. In response to the horizontal synchronizing pulse signal, a timer 3 produces a delay pulse D (FIG. 2C) which rises at the leading edge of the horizontal synchronizing pulse signal and has a width of about 7 .mu.s with the width being proportional to a delay control voltage. A gate pulse generator 4 generates a gate pulse E (FIG. 2D) of about 3 .mu.s which rises at the trailing edge of the delay pulse D.
Further, a burst separator 5 separates a color burst signal from the color video signal A and generates a color burst pulse signal B (FIG. 2B). A gate 8 generates a time base information pulse F (FIG. 2E) which rises at the leading edge of the gate pulse E and falls at the leading edge of the burst pulse B which in turn rises within the duration of the gate pulse E. Thus, the width of the pulse F varies corresponding to the phase relation between the gate pulse E and the burst pulse B.
A delay controller 22 applies to the timer 3 a delay control voltage in response to a change in the width of the pulse F, and the width of the delay pulse D varies in such a manner that the leading edge of a specific burst pulse is held for the entire duration of the gate pulse E. A sampling pulse generator 14 generates the pulse G (FIG. 2F) which rises at the trailing edge of the pulse F.
A stable reference horizontal synchronizing pulse is fed to a terminal 16. This pulse is produced in the video disc player. A sawtooth signal generator 17 generates a sawtooth signal K (FIG. 2G) in synchronization with the reference horizontal synchronizing pulse. A sample-and-hold circuit 18 produces an error signal L (FIG. 2H) corresponding to any time base variation by a sample-and-hold operation on the sawtooth signal K with the sampling pulse G. The error signal L is fed to a servo mechanism 20 through a servo amplifier 19. The servo mechanism 20 controls a detection device such as a tangential mirror employed in optical-type video disc players so that the leading edge of the specific burst pulse in color video signal is held at a point in constant phase relationship to the reference horizontal synchronizing pulse whereby the color burst time base is stabilized with respect to the reference horizontal synchronizing pulse and time base variations in reproduced color video signals are eliminated.
In the above prior-art device, since no color burst signal is present during the vertical retrace time of about 9 H (about 570 .mu.s), the width of the pulse F, which is the output of the gate 8, becomes equal to the width of the gate pulse E or may vary at random during the vertical blanking time. As a result, the delay controller 22 applies a faulty delay control voltage to the timer 3 and the width of the delay pulse D from the timer 3 becomes improper. Then, the error signal L from the sample-and-hold circuit 18 becomes faulty, and finally, undesired time base variations are produced. These are pulse-like variations having the sample frequency as the vertical synchronizing signal. This may not affect the video signals because of their occurrency during the vertical blanking time. However, periodical noise may appear as a distortion in the sound signal.